In a well intervention, one or more tools may be lowered into a wellbore. Tools may be placed into the wellbore by a variety of mechanisms. In wireline well intervention operations, a tool is lowered into the wellbore on a wire or cable which may include one or more electrical conductors. Typically, the wire is coupled to a winch at the surface. As understood in the art, a winch includes a drum around which the wire is wound. As the drum is rotated, the wire is extended or retracted (paid out or paid in respectively) depending on the direction of rotation of the drum. The position of the tool may thus be varied by operation of the winch.
In some operations in which the cable is to be put under higher tension, a capstan may be coupled to the cable between the winch and the wellbore. As understood in the art, a capstan or windlass includes one or more powered pulleys around which the wire may be at least partially wound. The capstan serves to apply force to the cable while reducing the tension at which it is wound onto or unwound from the winch. Thus, the capstan controls the position of the cable, while the winch follows the capstan to keep the cable wound on its drum.
In an operation utilizing a winch and capstan, the rotation of the winch and capstan are typically coordinated. A movement of the capstan without a coordinating movement of the winch may cause a decrease or increase in tension on the cable therebetween. A decrease in tension may allow the cable to slack and bow, possibly allowing the cable to slip through the capstan. On the other hand, an increase in tension may cause a rapid movement of the cable or the cable to break entirely, both situations being dangerous to equipment and personnel nearby.